The invention relates to an SMC for producing fibre-reinforced thermosetting components, a corresponding production process and a component produced from this SMC.
SMC stands for “Sheet Moulding Compound” and is a resin mat according to DIN 16913. In general, SMC refers to a flowable resin mat based on unsaturated polyester resin or vinyl ester resin and random fibre distribution in the plane of the mat. The reinforcing fibres customarily used are glass fibres. A typical SMC formulation consists of about 30% of polymer, about 30% of filler and about 30% of glass fibres, the remainder being composed of additives, such as, for example, colouring pigments, hardeners, dispersing auxiliaries, fillers and similar materials. SMC is generally produced as follows: the resin matrix is applied to two carrier films. These carrier films are drawn through an SMC machine and in the process transport the resin matrix, onto which the reinforcing fibres are sprinkled or laid. Once the reinforcing fibres have been deposited, the two films are pressed together to produce a kind of sandwich. This sandwich is transported through an impregnating section, which uses pushing and rocking movements to ensure that the fibres are uniformly wetted with the resin matrix. At the end of the machine, it is wound onto reels. Of crucial importance is a ripening process, which may be initiated by chemical and/or physical means. After this ripening process, the SMC can be further processed. After stripping off of the carrier foils, the SMC is customarily processed or pressed in heated steel moulds to form shaped parts.
The advantage of SMC is the high flowability, which has the effect that only 30 to 50% of the pressing mould has to be covered. The strength and stiffness can be varied in a wide range, depending on the reinforcing-fibre content.
As an alternative, it is known to place a woven fabric in the resin matrix for reinforcement. The disadvantage of this is that, although the strength is substantially increased, there is hardly any flowability. The pressing mould must be completely covered, which requires exact cutting to size, resulting in a lot of waste.
Furthermore, SMC with a glass-fibre reinforcement comprising both cut fibres (random fibres) and unidirectional fibres (UD fibres) is known. The UD fibres produce increased strength and stiffness properties in an axial direction and the random fibres determine the transverse strength. This SMC is preferably employed only for support-type components, such as, for example, bumper supports. It is not possible to produce sheet-like components because of the high tendency of the components to become distorted.